Establishing a transcriptional pathway for cell-fate and synaptic plasticity We hypothesize that the FOXO transcription factor controls motoneuron plasticity across lifespan in Drosophila. FOXOs are evolutionarily conserved proteins that coordinate cellular responses to developmental and environmental stimuli. Well known for their central position in molecular circuits regulating healthy aging and stress responses, their developmental functions have recently come into focus. In particular, FOXOs have emerged as important regulators of brain development. Neuronal functions of FOXOs have been investigated in mice, C. elegans, and Drosophila. To date, these functions include neuronal polarity, morphology, synaptic function, and memory consolidation. Though FOXO proteins are key regulators of multiple aspects of neuronal development and physiology, the neuronal-specific pathways in which they act are as yet undefined. Here we propose to analyze components of a novel neuronal FOXO pathway using combined molecular, genetic, and genome-wide approaches. We will test the hypothesis that FOXO activity is stimulated by Toll-6 signaling to inhibit apoptosis during embryogenesis and promote synaptic organization and plasticity during larval development. Mechanistic under- standing of FOXO's role in these processes requires the identification of its transcriptional targets. To this end, we propose an unbiased large-scale RNA-seq approach to identify the FOXO-dependent transcriptome. Thus, we propose an initial characterization of an entirely novel pathway, as well as a genome-wide screen for effector molecules. Together, these studies aim to define a novel neurotrophic pathway from cell surface to nuclear response in a powerful genetic model system. There is significant interest in modulating both the survival and synaptic functions of neurotrophic pathways in contexts as varied as neurodegenerative diseases, normal aging, and injury. The proposed genome- wide screens for effectors may suggest unexpected and novel players in these critical signaling pathways.